|Year : 2019 | Volume
| Issue : 1 | Page : 34-39
Naturally occurring radioactive material and naturally occurring mercury assessment of black powder in sales gas pipelines
Michael Ian Cowie1, AM El-Sherik2
1 Department of Environmental Protection, Saudi Arabian Oil Company, Dhahran 31311, Saudi Arabia
2 Research and Development Center, Saudi Arabian Oil Company, Dhahran 31311, Saudi Arabia
|Date of Submission||16-Sep-2018|
|Date of Decision||13-Mar-2019|
|Date of Acceptance||14-Mar-2019|
|Date of Web Publication||3-Jun-2019|
Michael Ian Cowie
Saudi Aramco, Dhahran 31311
Source of Support: None, Conflict of Interest: None
Black powder solids are formed inside the sales gas pipelines as a result of internal corrosion due to condensed moisture and the presence of corrosive gases, namely, H2S, CO2, and O2. Naturally occurring radioactive material, principally the radionuclides lead-210 and polonium-210, and naturally occurring mercury (NOM) (Hg) have been identified in black powder. A detailed investigation into both the radiological components and the presence of NOM associated with black powder was carried out by Saudi Aramco. The aim of the investigation was to determine if waste produced during pipeline scraping operations presented a workers protection or environmental control problem due to the radioactivity and mercury present. The investigation looked at Saudi Aramco's entire sales gas pipeline network and spanned scraping operations over a 3-year period. This article details the sampling and analysis methods used to assess black powder samples, the results of sample analysis and advice provided to ensure workers' protection, and environmental control of the waste produced during pipeline scraping activities.
Keywords: Black powder, mercury, naturally occurring mercury, naturally occurring radioactive material, pipeline scraping, sales gas, waste disposal, workers protection
|How to cite this article:|
Cowie MI, El-Sherik A M. Naturally occurring radioactive material and naturally occurring mercury assessment of black powder in sales gas pipelines. Radiat Prot Environ 2019;42:34-9
|How to cite this URL:|
Cowie MI, El-Sherik A M. Naturally occurring radioactive material and naturally occurring mercury assessment of black powder in sales gas pipelines. Radiat Prot Environ [serial online] 2019 [cited 2019 Aug 23];42:34-9. Available from: http://www.rpe.org.in/text.asp?2019/42/1/34/259672
| Introduction|| |
Black powder is a generic term given to corrosion products which become entrained in the flow of sales gas in transmission pipelines producing erosion failure in pressure control valves, blocking of instrumentation, and lowering the efficiency of process equipment. The constituent components of black powder have been reported to include iron sulfides, iron oxides, and iron carbonate in various combinations with additional contaminants including salts, sand, hydrocarbons, and metal debris. Some operators also report the presence of naturally occurring radioactive materials (NORM) and naturally occurring mercury (NOM) as part of the composition of their black powder. As such black powder could present a major health and environmental hazard. These hazardous substances require special procedures for handling and disposal of the removed black powder.
To mitigate the operational impact of black powder, routine scraping of sales gas pipelines is carried out to remove debris from the internal surfaces of the pipelines. These scraping operations bring workers into contact with the black powder, which when removed from the pipeline system becomes an industrial waste.
NORM is a byproduct of oil and gas production, with the progeny of radon, a particular concern in gas operations. A study to ascertain the extent and level of NORM in black powder debris from scraping activities associated with sales gas pipelines was established. The goals of which were to ensure that adequate protection was provided to workers recovering scrapers and handling black powder waste; the waste itself was disposed of in an appropriate manner. Saudi Aramco has a fully integrated NORM management strategy, a key component of which is to identify and dispose of any NORM waste above predefined exemption levels in a controlled manner. Saudi Aramco's preferred method of disposal for NORM waste is underground injection into a suitable geological formation by means of slurry fracture injection. Protocols for the identification and control of NOM have also been identified and incorporated into facility operating procedures.
The study presents and discusses lead-210 (210Pb) and polonium-210 (210Po) and mercury contaminants found in collected black powder samples.
| Representative Sampling|| |
The Saudi Aramco sales gas pipeline system is extensive and covers in excess of 6000 km, traversing the Kingdom of Saudi Arabia. Engineered into the pipeline system are scraper launch and receiver stations to facilitate the maintenance, inspection, and cleaning of the system. The receiver stations were used as locations to collect representative black powder samples on the recovery of pipeline scrapers after cleaning operations. It was planned to sample all receiver locations over a defined period of time, to determine levels of radioactivity present in black powder, and attempt to ascertain any geographical, temporal, or seasonal variations in radioactivity concentrations. However, operational constraints hampered the sampling plans and only a total of 65 black powder samples from different locations throughout the network have were collected and analyzed.
| Materials and Methods|| |
The analysis of the 210Pb and 210Po required extensive method development to attain satisfactory analyte recoveries from the black powder matrix. The main problem was to overcome the chemical interference of high levels of iron in the black powder acid extracts. The 210Pb method involved the deposition of polonium from dilute acidic solutions onto a silver disc, with the 210Po determined using alpha-spectrometry.,, Two methods were identified for the determination of 210Pb, which decays by low-energy beta and low-energy gamma (46.5 keV) emissions. The method of choice for the 210Pb analysis involved liquid scintillation counting, which was found to be suitable for the low levels encountered and provided acceptable sample throughputs.
Quality control of the analytical process included intercomparison studies with an internationally accredited analytical agency.
The following scheme summarizes the actual protocol of work used for the determination of 210Pb,210Po, and Hg in the “black powder” samples of this project.
For mercury determination, atomic spectroscopic methods such as atomic absorption spectrometry, inductively coupled plasma atomic emission spectrometry, and inductively coupled plasma mass spectrometry provide the required sensitivity for the measurement of mercury at the concentrations that are usually encountered in chemical analysis., Atomic absorption methods are widely used and employ the vapor generation techniques of excellent sensitivities for mercury and other several elements.
The following scheme summarizes the actual protocol of work used for the determination of 210Pb,210Po, and Hg in the “black powder” samples of this project.
[Figure 1] shows the analytical scheme used for measuring 210Pb, 210Po, and mercury in the black powder samples.
|Figure 1: The analytical scheme used for measuring lead-210, polonium-210, and mercury in the black powder samples|
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For sample preparation, around 2 g of sample were weighed in a watch glass and dried at 110°C for about 8 h before ashing (if the sample was oily paste, it should be dried first at 80°C for 5 h to remove volatile organics, followed by drying at 120°C for 8 h before ashing). The dry sample was transferred to a porcelain crucible and ashed in a programmable muffle furnace at 500°C for about 10–12 h. The muffle furnace was programmed to raise the temperature very slowly (from room temperature to 500°C within 10 h) to avoid combusting the sample inside the furnace in the temperature range 250°C–350°C. After cooling to room temperature, the ash sample was transferred quantitatively to a 250-mL conical flask and digested by concentrated hydrochloric acid (HCL) on a hotplate stirrer in the fume-hood. The flask was removed from the hotplate and allowed to cool. The content of the flask was transferred to a 50-mL centrifuge tube and centrifuged for 5 min at 300 rpm. The supernatant was decanted into a clean labeled 250-mL measuring flask. The residue was washed with 50 mL of dilute HCl and centrifuged again. The supernatant was decanted also into the same labeled measuring flask. The residue was transferred to a Teflon beaker and digested with concentrated nitric acid/hydrofluoric acid, (HNO3+ HF), perchloric acid, and HCl for complete dissolution. The solution was evaporated to near dryness and 3 mL concentrated HCl was added and evaporated again to near dryness to expel HF and change the medium to chloride form. The last step was repeated to assure complete change. The content of the Teflon beaker was dissolved in dilute HCl and transferred also quantitatively to the same labeled measuring flask. The dissolved sample in the labeled measuring flask was diluted to the volume by dilute HCl. Two grams of the samples were completely dissolved in 250 mL and ready to be submitted to the radiochemical procedures to determine 210Pb or 210Po.
210Po was determined by α-spectrometry. Five mL of the solution from the labeled measuring flask was transferred to a clean beaker containing 100 mL of 0.5 M HCl and spiked with 100 μL of 209Po radiotracer (~1.2 dpm). Due to the very high content of ferric ions in the samples, the beaker was heated to near boiling and drops of saturated ascorbic acid solution were added slowly with stirring to reduce ferric iron to ferrous (this can be confirmed by observing the change in the solution color from faint yellow to colorless). This addition step was key for analyzing210Po in this kind of sample. The solution was placed in a modified plating cell and the polonium isotopes were allowed to deposit spontaneously on a silver disc for 4 h at 90°C–98°C with continuous stirring. Water bath was used to increase the temperature of the sample solution in the plating cell. The silver disc was removed carefully, rinsed with distilled water, dried, and left for 1 h after plating (to allow for decay of short-lived polonium isotopes). The silver disc was ready for counting using a high-resolution α-spectrometer. The prepared silver disc was counted using the 4.866 MeV and 5.305 MeV alpha peaks of 209Po and 210Po, respectively. For samples of expected low activity, the sample was counted for a total elapsed time to get a statistically clear peak for 210Po.
The lower limit of detection for 210Po was 0.079Bq/g.
210Pb content was determined by liquid scintillation, Ten mL of the dissolved sample solution from the labeled flask was added to 200–300 mL of water. Nineteen mg of barium nitrate were added and stirred to dissolve. The pH was adjusted to 2–3 by addition of sodium hydroxide solution. The solution was boiled and 6 mL of 1M H2 SO4 was added drop by drop with stirring (Ra and Pb were coprecipitated with Ba as sulfate). The sample was left to warm for 15 min on the hot plate without stirring to develop the precipitate. The sample was cooled, centrifuged, and the supernatant was discarded. The precipitate was washed to about neutral (pH >6) by distilled water. Four mL of 0.25M alkaline ethylenediaminetetraacetic acid solution was added, and the sample was warmed in a water bath for complete dissolution (recovery is 95%–100%). The sample was gently evaporated to 2–3 mL, cooled to room temperature, and mixed with 15 mL of OptiPhase HiSafe 3 liquid scintillation cocktail. The sample was cooled for 3 h in the fridge, then was well-shaken and measured using liquid scintillation spectrometer using α/β discrimination counting mode.
The lower limit of detection for 210Po was <0.01Bq/g.
Saudi Aramco has established levels below which material is considered exempt from control as NORM, the exemption level for radionuclides of concern in sales gas pipelines are detailed in [Table 1].
| Results and Discussion|| |
Over 70% of the samples collected indicate levels of 210Pb and 210Po above the exemption level, and therefore, would be regarded as having enhanced levels of NORM. [Figure 2] and [Figure 3] show the activity concentrations of 210Pb and 210Po, respectively. The results also indicate that210Pb and 210Po are in radioactive equilibrium, which would require a time period in excess of 2 years. As scraping activities are carried out at frequencies <2 years, further investigation is required to fully understand the mechanism, by which NORM is incorporated into the black powder matrix.
|Figure 2: Lead-210 concentration in black powder samples from sales gas pipelines|
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|Figure 3: Polonium-210 concentration in black powder from sales gas pipelines|
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Relationship between polonium-210 and lead-210
The relationship between210Po and 210Pb was explored. As can be seen in [Figure 4], there was a close correlation (r2 = 0.9881) between the two parameters. This would indicate that the two radionuclides were in secular equilibrium. This result can be used to advantage in the future studies, since measurement of 210Pb, a relatively straightforward and more rapid analysis compared to210Po, could be used as a good indicator of 210Po levels in the black powder.
Determination of total mercury
The black powder samples were prepared in the following manner, triplicate 0.20 g portions of the sample were weighed into 50 mL hot block vessel and 5 mL of deionized water (DI), 3.75 mL of concentrated HCl, and 1.25 mL of concentrated HNO3 were added to the vessel. The sample was heated for 2 min at 95°C in the hot block. The sample was allowed to cool and the volume was adjusted to 30 mL with water. Three mL of 5% potassium permanganate was added. A lid was put on the hot block vessel and placed in the hot block digestor for 30 min at 95°C. The vessel was cooled and 3 mL of sodium chloride-hydroxylamine hydrochloride was added. In case a clear solution was not obtained, additional sodium chloride-hydroxylamine hydrochloride solution was added until a clear solution was obtained. The sample volume was adjusted to 50 mL with DI and transferred to an autosampler tube.
The sample was then measured in the following manner, a flow injection mercury system (FIMS) analyzer was turned on and the lamp was allowed to warm up for at least 30 min before beginning analysis. After pump tubes were placed in the stannous chloride and HCl solution containers, these reagents were automatically transferred (pumped) to the instrument through the transfer lines. The samples were analyzed using WinLab FIMS software to analyze batches of prepared samples. Samples were automatically mixed with the reagents in the instrument to cause the reduction of mercury in the sample. The resulting mercury vapor was subsequently analyzed by the FIMS. To determine the mercury result, the following formula was used:
Mercury result (mg/kg) = ([Instrument reading/1000] × 50)/sample mass
Determination of mercury by toxicity characteristic leaching procedure
The toxicity characteristic leaching procedure (TCLP) (US Environmental Protection Agency Method 1311) is used to establish whether a solid waste is hazardous, based on specified limits for selected metals. The TCLP data also provides indication of the relative environmental mobility of selected metals.
Solid samples were extracted with an amount of extraction fluid equal to 20 times the weight of the sample. The extraction fluid employed was determined based on the alkalinity of the solid phase of the waste. Following the extraction, the liquid TCLP extract was separated from the solid phase by filtration through a 0.6–0.8 nm glass fiber filter. Total mercury (organic and inorganic) was measured in this extract as mentioned above.
Forty-seven black powder samples (72% of the samples tested) showed total mercury concentrations in excess of >4.0 mg/kg which requiring that they are tested further following TCLP. The TCLP extractable mercury concentrations ranged from <0.0002 to 0.99 mg/L, with a mean value of 0.034 mg/L. Only two samples had TCLP mercury concentrations that exceeded the action level of 0.2 mg/L. While the total mercury concentrations were significant, the mercury appeared to be in an inert form and not readily extractable under TCLP conditions, which provides indication of environmental mobility.
| Workers Protection and Contamination Control|| |
Pipeline scraping activities have been identified as an operation where workers can come into direct contact with potentially enhanced levels of NORM and NOM, and that radioactive contamination can be spread in the immediate and surrounding work area, as such generic advice is provided to ensure workers' protection and control the spread of contamination. This advice includes the following requirements:
Personnel required to work with NORM must be trained in the associated hazards.
- All NORM operations shall be covered by a safe system of work, which shall identify the hazards and highlight the precautions to be taken
- Any item or area with detectable levels of loose NORM contamination shall be subject to radiological controls
- Appropriate personal protective equipment shall be worn (which may include but not be restricted to).
- Tyvek coveralls
- Neoprene, polyvinyl chloride, or nitrile butadiene rubber gloves
- Half-face respirators with high-efficiency particulate air (HEPA) cartridges; these must be fit tested
- Quarter-face HEPA disposable respirators.
- Eating, drinking, smoking, and chewing gum are not allowed in work areas where potential NORM contamination exists
- Only essential personnel shall be allowed in the work areas where potential NORM contamination exists
- Personnel shall thoroughly wash up with copious quantities of soap and water, after working with contaminated equipment, and before eating, drinking, or smoking, and at the end of the workday.
All NORM operations shall be carried out in a manner which prevents the spread of NORM contamination and minimizes the potential for workers to be exposed to NORM.
NORM operations shall only be undertaken in areas which are clearly demarcated, and access is restricted to those directly involved in the operations.
Advice is provided that waste debris from scraping activities is required to be contained and stored in suitable receptacles pending its NORM status being determined.
| Conclusions|| |
Levels of 210Pb and 210Po above the Saudi Aramco exemption levels have been found in 70% of the black powder samples collected.
210Pb and 210Po appear to be in radioactive equilibrium in the samples analyzed.
Further sampling is required to fully characterize the extent and spread of NORM contamination in the gas pipeline system.
There was poor correlation between the total activity and total mercury content, indicating different pathways.
Mercury appeared to be in an inert form and not readily extractable under TCLP conditions.
Worker's protection and contamination control procedures have been established.
Over 70% of the debris from scraping operations may be considered NORM and NOM waste and disposed of by appropriate disposal option.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
| References|| |
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Saudi Aramco Engineering Procedure, SAEP-0358. Management of Technologically Enhanced Naturally Occurring Radio-active Material (NORM); 2005.
[Figure 1], [Figure 2], [Figure 3], [Figure 4]